In a copending application of Norman L. Stauffer, Ser. No. 085,821, filed Oct. 17, 1979, a low cost auto focus system was shown wherein as few as two radiation responsive detector pairs were mounted behind an auxillary lens and a pair of lenslets so as to produce four signals identified as a.sub.1, b.sub.1, a.sub.2 and b.sub.2 which were operated upon to produce an output signal V that varied in accordance with the equation: V=.vertline.log a.sub.1 /b.sub.2 .vertline.-.vertline.log a.sub.2 /b.sub.1 .vertline..
In this copending application, it was shown that V would normally vary from a positive value when the auxillary lens was at an infinity position to a negative value when the auxillary lens was at a "near" position with the proper focus position being at the point where the output voltage crossed the zero axis. A suitable coupling was provided between the auxillary lens and the taking lens of a camera so that with proper detection and motive apparatus the taking lens could be automatically placed at the proper focus position with respect to the camera film.
For most photography, the scenes being viewed have a significant contrast and the output signal V in the prior art system is a fairly large positive signal at infinity, a fairly large negative signal at the "near" position and a fairly steep slope to the curve near the zero cross-over position. Thus, for most scenes, the detection of the zero cross-over point is quite easily obtained and the auto focus position calculated in a satisfactory manner. It has been found, however, that for very low or 0 contrast conditions, the size of the output signal V may be so small that variations in the parameters of the system, such as a mismatch of the detectors, can cause improper zero cross-over situations to occur. As was explained in the above-referred to copending application, when the proper focus position cannot be readily obtained, it is best for the system to focus at the infinity or the hyperfocal distance, but when a low contrast situation occurs, the zero cross-over point may occur at almost any point including the "near" position in which case an unsatisfactory focus may be obtained.